1,720,966 research outputs found
Nano LEGO®
No full textSelf-Assembly has predominantly been nature's unique tool for building remarkable entities. This thesis shows how this versatile tool can be utilised for building devices that are useful to mankind. Starting with advanced fuel cells which produce clean energy in a highly efficient way; continuing on to carbon nano networks which are one of the strongest materials known to man; and finally supermagnets which have the potential to revolutionise the medical and electronics industry.Chemical EngineeringApplied Science
Slip and friction of liquid flow over solid surfaces: On the validity of the no slip condition in hydrodynamic systems
No full textIn the first chapter a review is presented of the existing ideas on surfactant dynamics, and irreversible thermodynamics is introduced as a tool of reference for different models. The two examples of lubrication and detergency are used to survey the implications of specific boundary conditions in dissipative processes. The second chapter gives a review of some published results and reinterprets those in terms of entropy production. The concept of a critical capillary radius, for which liquid flow with slip in the interface would be the thermodynamically favoured mode is discussed. This discussion leads to a new interpretation of the slip length : as a measure for the interfacial viscosity. To investigate this concept, Newtonian surfactant solutions are subjected to oscillating flow through a membrane. Phase differences between the flows and their driving forces are recorded as a function of the stress. The observations are attributed to the complexity of the pore/solution interface, where surfactant adsorption is taking place. The experiment indicates viscoelastic interfacial behaviour. With low shear stress as the driving force surface elasticity is predominant and the no slip boundary condition applies for the flow pattern. At higher stresses, surface motion is allowed. In the fourth chapter an expression is worked out for the momentum transfer from the bulk of a flowing liquid to the interface. This equation allows the calculation of the slip length as a function of the relevant driving forces. Two linear regimes are observed in flows through a membrane of surfactant solutions with different concentrations. The difference between these two regimes is described in terms of a stick to slip transition. In the following chapter, the mathematical description is extended to consider both the stick to slip transition and the e¤ect of coupled hydrodynamic and electroosmotic flows. This extended model is then applied to, as yet, unexplained observations of regime transitions in the electroosmotic flow through porous plugs. It follows from the analysis that the slip length (i.e. the interfacial viscosity) should depend on surface forces, which are characterized by the zeta potential. In the sixth chapter our mathematical models are applied to describe the flow of various surfactants at different concentrations through porous membranes. A definition of the deceivingly simple concept of a slip length as the sum of interfacial effects is suggested. These effects can then be lumped together in a new interfacial excess function for the fluid flow over a solid surface (the viscoelastic interfacial flow). A description in terms of a (complex) interfacial viscosity is preferred over the concept of a slip length. The last chapter summarizes our results in succinct statements about the physical chemistry of surfactant solutions, which flow over solid pore walls of membranes, textiles and sorbent materials.Applied Science
Modelling and Numerical Simulations of a Membrane Electrode Assembly for Fuel Cell Applications: A hydrogen crossover study based on Non-Equilibrium Thermodynamics
No full textIn this study a one-dimensional, steady-state, non-isothermal numerical model was developed in order to investigate the transport phenomena occurring in a membrane electrode assembly for fuel cell applications and to provide an insight on the effects of the hydrogen crossover. The hydrogen that permeates through the membrane is consumed without generation of useful work. Moreover, the effects caused by the hydrogen crossover are so far unclear. This complex phenomenon is usually neglected in fuel cell modelling and the results of experimental measurements are often not in agreement. The proposed model consists of a set of transport equations, based on non-equilibrium thermodynamic. The latter derives from irreversible thermodynamics and it provides a systematic way to study heterogeneous systems, like fuel cells. The numerical results are thermodynamically consistent with the second law of thermodynamics. Initially the isothermal boundary condition was applied to the system in order to first study the coupled charge and mass transport, including the hydrogen crossover. However, it was found out that the thermal effects are not negligible in the simulated system and thus the constant temperature assumption was removed. The model described with the NET method provides useful information about the driving forces profiles (total pressure, hydrogen partial pressure, electric potential and temperature) and entropy production. This allows a more accurate estimation of non-constant parameters and also a more reliable prediction on the hydrogen crossover flux over a wide range of operating conditions. An optimization procedure was carried out showing that even though the set of optimal parameters produced an increment of almost four times in crossover, the power output increased without being affected by the variations hydrogen permeation. Although the results of the simulations show that hydrogen crossover has no significant effects on fuel cell performance in term of power output, the adopted method of non-equilibrium thermodynamics permitted to identify that part of the reduction in cell voltage in near open circuit conditions is attributable to the hydrogen crossover. The effects of hydrogen permeation are only noticeable at very low current densities, so when the crossover flux is comparable to the hydrogen reacting at the catalyst layer. A final analysis was carried out with increased permeability (ten, fifty and a hundred time). This verified that considerable hydrogen crossover flux lead to effects on net heat flux, entropy production, temperature distribution and power output.Applied SciencesSustainable Energy TechnologyChemE/Chemical Engineerin
Conductive Graphitic Networks: From Atoms to Fuel Cells
No full textGraphitic materials have attracted a great interest in the field of sustainable energy production and storage because of their excellent electrical, mechanical and chemical properties. This thesis modestly contributes to this global research by investigating new interconnected carbon nanostructures, here called Carbon Nano-Networks (CNNs). The work is divided into two parts. The first part deals with the synthesis of CNNs consisting of Chemical Vapor Deposition (CVD) of ethene over metal catalyst nanoparticles (NPs) synthesized in bicontinuous microemulsions (BMEs). Chapter 1 focuses on the characterization of dense microemulsions, both experimentally and computationally, using a coarse-grained molecular dynamics simulation tool. Bicontinuity of microemulsions is visualized. Chapter 2 describes the synthesis of NPs in BMEs. The effect of the precursors and of the microemulsion composition on the size, polidispersity and stability of the NPs is analyzed. Finally a mechanism of formation of NPs in BMEs is proposed. Chapter 3 investigates the synthesis of CNNs via CVD of ethene over metallic particles synthesized in BMEs. The effect of synthesis parameters on the final structure is studied. Properties of CNNs, such as porosity and conductivity are investigated. The second part deals with the use of CNNs as catalyst support in Polymer electrolyte membranes (PEM) Fuel Cells. Chapter 4 gives a brief overview of PEM Fuel Cells basics, materials and challenges. In Chapter 5, activity and durability of Pt deposited over CNNs is compared to Pt over carbon nano-tubes and to commercial catalyst. In Chapter 6, CNNs are used as support for non-noble metal catalyst. Performances are evaluated in-situ and ex-situ. Chapter 7 deals with an innovative manufacturing technique for an electrode: CNNs are grown directly over carbon paper. Resistance to corrosion as a function of synthesis parameters is evaluated. Pt is electrodeposited over the synthesized electrode support, and its activity and durability is evaluated and compared to commercial catalyst. The results presented in terms of cost, activity or durability are either superior to commercial catalyst or of the same order of magnitude of state-of-the-art catalyst. Nevertheless, the simplicity of CNNs synthesis procedure, the low price of catalyst precursor and the reduction of manufacturing steps make this novel electrode promising as material for fuel cells. In conclusion, the work described in this thesis certainly does not lead to immediate improvements in efficiency of fuel cells but it does provide for new and potentially more sustainable material solutions with which it may well be possible to attain these improvements in the near future.Chemical EngineeringApplied Science
Optimal ionic strength for nonionically initiated polymerization
No full textSurfactant-free emulsion polymerization involving a nonionic, and hence uncharged initiator presents a new approach towards environmentally friendly procedures to synthesize latex particles. Under optimal solvent conditions, notably pH and ionic strength, the latex particles are stabilized by the natural development of ionic charge at the surface of the particles. We emphasize that the present process does not at all involve the addition of stabilizers such as surfactants or the creation of surface-active species from ionic initiators. The width of the size distribution is found to vary strongly with experimental conditions, notably the ionic strength and to a much lesser extent pH. The phenomenon is explained by a critical ionic strength dependence of the aggregation of the just nucleated primary particles into larger secondary particles, the so-called “coagulative nucleation” step.Chemical EngineeringApplied Science
Properties of advanced (reduced) graphene oxide-alginate biopolymer films
No full textIn this work, properties of Calcium alginate-reduced graphene oxide and Barium alginate‐reduced graphene oxide composite films are explored. In addition, the properties of the divalent metal ion-cross-linked alginate composite films are compared to the analogous properties of uncross‐linked Sodium alginate-graphene oxide composite films of the corresponding compositions. As the filler, used in the preparation of the composite films, is obtained by chemical oxidation of graphite, the prevailing knowledge of the process coupled with in situ X-ray diffraction investigation of the samples prepared by such a method is presented as well.<br/
Low Noble Metal Content Catalysts for Hydrogen Fuel Technology
No full textThe increasing energy demand of the world population in combination with tangible climate change effects stemming from rising carbon dioxide emissions is currently characterizing a large portion of the political and societal debate. Despite huge technological advancement in the field of renewable energy resulting in energy prices lower than that of fossil based energy, the rate of greenhouse gas emissions has not even levelled off but rather kept increasing. A part of the problem lies in the very nature of season and weather dependent energy conversion technologies producing electricity peaks that are hard to buffer. The solar and wind powered scenario is not yet able to completely replace the relatively demand flexible fossil fuel based power plants. The gap between energy production and energy use, in essence meaning storage and distribution of sustainable energy, constitutes one of the largest challenges of our times. Hydrogen has been proposed as a molecule with the potential of being an important energy carrier in a renewable energy based economy. In a fuel cell, hydrogen can be electrochemically oxidized to water, releasing its chemical energy without the emission of combustion by-products like carbon dioxide. Commonly platinum is used as a catalyst to speed up the anode and cathode reactions in a fuel cell. Reversibly, an electrolyser uses electricity to electrochemically split water into its constituents; hydrogen and oxygen. Ideally, hydrogen could be produced where and when the electricity is available or cheap and be stored or transported to the location where it is needed, although technical challenges as well as infrastructural hurdles are still to be solved. If electrochemical devices, such as fuel cells, are to play a major role in the future energy landscape a better understanding of catalytic processes along with cheap and scalable non-noble metal catalysts are still needed.ChemE/Advanced Soft MatterRST/Storage of Electrochemical EnergyChemE/Chemical Engineerin
Towards low-cost PEM fuel cells: Interfacial effects and material dynamics of a non-PGM electrocatalyst
No full textIn this thesis several aspects of PEM fuel cells are discussed. From an atomic scale, e.g. crystal and electronic structure of a non-noble metal material, e.g. (H)zLiMn2O4 (protonated spinel lithium manganese oxide) for electrocatalysis and its dynamics, to meso-macro scale effects such as those concerning transport resistances at the membrane/electrode interface.RST/Storage of Electrochemical EnergyChemE/Advanced Soft Matte
Can rheological behaviour of alkyd emulsions be measured with Diffusing Wave Spectroscopy?: A numerical study of the potential of Diffusing Wave Spectroscopy to measure rheological changes in films of drying alkyd resin emulsion paints
No full textDue to legislative changes, the usage of indoor paints containing organic solvents is prohibited. An alternative for these paints is waterborne paint. However, these paints do not yet perform as well as the conventional paints concerning rheological behaviour, open time and storage life. This project focuses only on the rheological aspects of the assessment of the quality of paints…Applied SciencesPhysical Chemistry and Molecular Thermodynamicsthe Coating Progra
Electrode compartment for an electrochemical cell, a refreshing system for it and an emulsion to be used therefore
No full textThe invention relates to an electrode compartment for an electrochemical cell, including a bicontinuous micro-epulsion, wherein catalytic parts are generated in-situ in a fluid, which can act as a cathode as well as an anode. The electrode compartment comprises a connection to supply fuel or an oxidator, for example oxygen, to the compartment. The electrode compartment is part of a refreshing system with a reserve container for an emulsion and a storage container for used emulsion, conduits to connect each of the containers with the electrode compartment and a transport unit, for example a pump, to move the emulsion.DelftChemTechApplied Science
- …
